Hazards
Avoid contact with the liquid and vapor of the chemicals in this demonstration.
Chemicals and Solutions
- 100 ml of polyurethane a (toluene diisocyanate plus extras)
- 136 ml of polyurethane b (gycerol plus extras)
Materials
- large plastic beaker (2 L capacity)
- stirring rod
Procedure
Mix components a and b in the beaker. Stir until the mixture starts to foam.
Discussion
The polyurethane forms when an isocyanate group (R-N=C=O) from toluene diisocyante reacts with a hydroxyl group from glycerol forming an ester-like bond. The reaction can proceed in three directions since all three hydoxyl groups of the triol can react and both isocyanate groups can react.This leads to a large molecule that is rigidly held into a three dimensional structure.
The foaming is caused by the small amount of water that is present. The water decomposes isocyanate:
\( \ce{ R-N=C=O + H2O -> RNH2 + CO2 } \)